In a typical slurry polymerization reaction, monomer, diluent and a particulate catalyst are fed to a reaction zone where the monomer is polymerized. The diluent does not react but the quantity of diluent supplied to the reaction zone is typically utilized to control solids concentration in the reaction zone and also to provide a convenient mechanism for introducing the catalyst into the reactor.
Slurry loop reaction zones are in the form of continuous loop in which polymerisation occurs in the circulating slurry. Inlets are provided for the monomer and diluent and for catalyst so that they can be introduced into the reaction zone. Slurry is removed from the reactor through at least one discharge conduit and further reactants added through the inlets.
The catalyst is introduced into the reaction zone using an injection nozzle which is usually a narrow tube which projects into the reaction zone. Such is described, for example, in U.S. Pat. No. 3,694,423.
The projection into the reaction zone assists in ensuring that the catalyst is introduced into the centre of the reaction zone and into the middle of the flow of slurry. By injecting into the more central areas of the reaction zone the catalyst is generally introduced into a zone of well-defined flow from where it is quickly and evenly dispersed into the reaction mixture, and in a zone away from the walls of the reaction zone.
The catalyst is of high activity and great care has been taken to ensure that the catalyst, which is relatively concentrated at its point of injection, is quickly dispersed away from the tip of the injection nozzle to prevent fouling thereof. Numerous methods have been described which attempt to avoid the problem of fouling of the injection nozzle itself. U.S. Pat. No. 4,698,211, for example, relates to a process in a stirred tank wherein the injection nozzle is located in the same horizontal plane as an impeller to ensure rapid dispersion. In other techniques to avoid the fouling of the injection nozzle, it has been proposed to use an injection nozzle which is provided with one or more conduits around the central tube, and flowing gases or diluent into the conduits which act to “shield” the catalyst from the reaction mixture at the point of entry into the reaction zone. Reference may be made, for example, to WO 2008/042182.
Whilst this has helped to prevent fouling at the tips of the injection nozzles in slurry reactors it nevertheless remains that these nozzles represent protrusions into the reaction zone. Protrusions into the reaction zone can affect the uniform flow of slurry and can also have a propensity to fouling by formation of deposits thereon. These deposits can form and then fall off of the protrusions leading to agglomerates in the reaction zone. It is generally desired to minimise such fouling. However, to date the requirements for introducing and dispersing the catalyst have generally over-ridden any concerns about the protrusion of the injection nozzle into the reactor.